Research ArticleElectroplasticity in the Al0.6CoCrFeNiMn high entropy alloy subjected to electrically-assisted uniaxial tension
Graphical abstract
Introduction
Rapid developments in the aerospace industry have created a demand for alloys with superior properties. One option that provides a novel design method for materials [1, 2] is high entropy alloys (HEAs), a new class of materials composed of multi-principal elements with excellent strength [3], [4], [5], superior fracture toughness [6], exceptional irradiation resistance [7], corrosion resistance [8], and wear resistance [9]. The face-centered cubic (FCC)-structured HEAs (e.g., CoCrFeMnNi HEA) exhibit excellent ductility, but their strength is generally not sufficient for engineering applications [10]. To improve the strength-ductility balance, Aluminum, a body-centered cubic (BCC) phase stabilizer with low density, has been added to inherently FCC-structured HEAs to enhance their strength by forming hard BCC phases [11, 12]. For AlxCoCrFeNiMn HEA, a single FCC structure changes to an FCC + BCC structure in the range of 8 at.%–16 at.% of Al, and it transforms to a single BCC structure as the Al content is larger than 16 at.% [13]. Among them, the dual-phase HEAs have high strength but are difficult to form at room temperature [13], [14], [15]. Elevated temperature processes such as hot working or warm forming were adopted to improve the formability of materials [16]; however, long-term exposure to high temperature led to serious oxidation of components and adhesion between the mold and billet.
Electrically assisted deformation (EAD), which reduces flow stress by applying current to metals during deformation [17, 18], can significantly improve the formability of the components [19] and has been applied to the forming of aluminum alloys [17], [18], [19], [20], magnesium alloys [21, 22], and copper alloys [23, 24]. The elongation of aluminum alloy can be increased by as much as 400% by adjusting the electrical pulse parameters [25]. The rotation of a crystal lattice in the AZ31B magnesium alloy was affected by the pulse current during EAD [26]. In these cases, the grains rotate from the basal hard orientation to a softer orientation, thereby weakening the basal texture [27] and resulting in extreme dynamic softening of the stress–strain curves before necking. In addition, the pulse current can induce recrystallization and increase the plasticity of deformed copper alloys [24].
Up to now, the detailed mechanisms of electroplasticity are not yet clear due to the difficulty of decoupling the thermal and electrical effects. Rudolf et al. [28] attributed the mechanical properties of pure copper and iron under EAD to the Joule heating effect induced by free electron transport since there are no other factors reducing flow stress. Zhang et al. [29] confirmed the existence of electron force in the EAD process by observing the dislocation annihilation and regeneration in nickel-based superalloys using electrical in situ transmission electron microscopy and found that electron force can affect the mechanical properties of alloys by promoting dislocation motion, i.e., the electron wind theory. Liu et al. [30] concluded that electroplasticity results from local Joule heating and atomic bond weakening near the defects. The thermal and mechanical models of AZ31 magnesium alloy under EAD established by our team [31] have shown that Joule heating is the main factor affecting the deformation behavior under EAD, of which 10%, or even less, is contributed by the electrical effect. In addition, researchers have proposed other theories such as thermal expansion, skin effect, and magnetoelastic to explain electroplasticity behavior [32], while the Joule heating effect and the electron wind theory are still considered the main mechanisms [33]. It can be found from the previous studies that there are obvious differences in the deformation mechanisms of various alloys under pulse current.
Compared with conventional dilute solid solution alloys (DSSAs), HEAs contain more elements, which condition induces the high entropy effect, the lattice distortion effect, the sluggish diffusion, and the “cocktail” effect [34]. Previous studies have confirmed that these characteristics of HEAs will lead to Portevin-Le Chatelier (PLC) effect at lower temperatures under direct current (DC), resulting in a decrease in the elongation of HEA [35, 36]. All five solid solution atoms in the cantor alloy are activated and cooperative diffused, which is significantly different from traditional DSSAs [36]. The current studies are limited to the deformation behavior of single-phase FCC alloys under DC, and there is a lack of detailed studies on the mechanisms for the electroplasticity of HEAs under pulse current that may produce greater elongation than that under DC. Hence, whether the formability of HEAs can be improved by EAD needs further study, and it is of great significance to study the electroplasticity of HEAs for forming HEA components in the aerospace field.
In this work, the electroplasticity of the dual-phase Al0.6CoCrFeNiMn alloy was investigated from the perspective of microstructural evolution and tensile behavior. The mechanical properties of the HEA were first obtained using uniaxial tension subjected to various current densities, and the influence of current density on the tensile behavior of this alloy was analyzed. The specimens with the greatest elongation (30 A mm–2) and with obvious strain softening (60 A mm–2) were selected to compare with the tensile specimen at room temperature (RT). Based on the difference in lattice structure between HEAs and DSSAs, the electroplasticity of the HEA as well as the effect of local Joule heating on tensile behavior during EAD were determined and discussed.
Section snippets
Material
The Al0.6CoCrFeNiMn HEA was produced by vacuum induction melting using pure elements (99.9 wt%) as raw materials, and the ingots were homogenized at 1100 °C for 24 h in a vacuum furnace. After that, the blocks obtained were rolled at 900 °C to achieve a thickness reduction of 60%. The microstructure of the Al0.6CoCrFeNiMn HEA before tension is shown in Fig. 1. Fig 1(a) shows that the alloy is composed of the FCC phases and the BCC phases. The protruded phases are BCC phases (marked in green)
Joule heating temperature
The temperature field of the Al0.6CoCrFeNiMn HEA during the EAD with different current densities is shown in Fig. 3. During the EAD process, the variation of temperature can be divided into four typical stages (Fig. 3(a)), and the inset images represent (I) the temperature distributions before tension, (II) initial steady-state temperature, (III) crack initiation, and (IV) crack propagation stages. The temperature increases rapidly from the initial room temperature (Stage I) as the current is
Discussion
Electroplasticity mechanisms mainly involve Joule heating theory [48, 49] and the electron wind effect [27, 50]. The core of the electron wind effect is that the incident drifting electrons provide an additional force to promote dislocation movement by acting on the dislocation directly [51]. This theory was confirmed by the EAD of conventional alloys [32, 52]. Joule heating theory is based on the energy transformation caused by free electron transport. For conventional alloys, as electrons
Conclusions
The electroplasticity of the Al0.6CoCrFeNiMn HEA subjected to electrically assisted tension was systematically studied. The effect of pulse current on the tensile behavior and microstructural evolution of the HEA was analyzed. The difference in electroplasticity between the HEAs and DSSAs was concluded. The following conclusions were drawn.
- (1)
Joule heating is generated by applying pulse current to the samples, and peak temperature increases with increasing current density. The temperature
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (No. 51635005) and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. NRF-2021R1A2C3006662). Zhiqin Yang is supported by the China Scholarship Council (CSC, No. 202106120151).
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